Cutting tap and method of making same

ABSTRACT

A method of making a cutting tap includes the steps of: grinding a blank to form a threaded body portion at an axially forward end of the cutting tap, grinding one or more flutes in the threaded body portion to form cutting edges; grinding the threaded body portion to form a first cutting thread and a second cutting thread, the first cutting thread at a first distance from the axially forward end of the cutting tap, and the second cutting thread at a second distance from the axially forward end of the cutting tap; and grinding a chamfer in the threaded body portion such that a thickness of a section of material removed from the second cutting thread is smaller than a thickness of a section of material removed from the first cutting thread during a tapping operation.

CROSS-NOTING TO RELATED APPLICATIONS

This Application is related to application Ser. No. 11/582,805, entitled“Cutting Tap and Method of Making Cutting Tap”, filed Oct. 18, 2006, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates in general to a cutting tap, and in particular toa cutting tap having a cutting edge geometry that improves theresistance of the cutting edges to chipping and fracture.

Mechanisms and machine components requiring screw threads have a longhistory in technology. Specifically, the application of screw threads asfastener components dominates over all other means to join parts intoassemblies. Although there are many ways to generate screw threads bothinternal as well as external, experience has shown that taps are thefavored means to generate the internal screw thread. There currentlyexist two tapping methods to generate internal screw threads. Thedominant tapping method is by cutting and removing material from thewalls of a hole to produce a helical V-shaped screw thread.Alternatively, internal screw threads can be created by displacingmaterial to form an internal screw thread. However, tapping by cuttingmaterial is generally favored because this method requires lower torqueand produces a more perfect thread form.

The dimensional accuracy of the shape and size of the internal screwthread controls the precision and fit of the screw thread assembly.Additionally, the speed of tapping affects the cost to produce aninternal screw thread.

There are two materials used to manufacture cutting taps. High-speedsteel is widely used for taps because of its high strength. However,cemented tungsten carbide is favored as a material for manufacturingother cutting tools over high-speed steel owing to properties such ashigher hardness and high temperature stability including the ability toretain hardness at high temperatures. Typically, cutting toolsmanufactured from cemented carbide can be used at cutting speeds thatare at least three times higher than tools manufactured from“high-speed” steel and the life of the tool is longer.

Referring now to FIGS. 9-11, there is shown one flute of a four-flutedprior art cutting tap that has a straight cutting face. In general, thecutting tap generates an internal thread form by a succession of cuttingedges on the chamfered section of the tap having a length L. Material isremoved from the wall of the hole until the final thread form isobtained with the first full thread on the main body of the tap. Thisprogressive formation of an internal thread is illustrated in FIG. 9 bysuperimposing the sections of material removed by each of the fourflutes.

As shown in FIG. 10, the prior art, cutting tap has a straight cuttingface that is inclined relative to a radial reference line that travelsfrom the cutting edge at the major diameter to the center of the cuttingtap at a cutting angle (or rake angle) A1. In FIG. 10, the cutting angleA1 is defined as the included angle between a line passing along thesurface of the cutting face and the radial reference line. The cuttingangle A1 is positive when the inclination from the radial reference lineis in the counterclockwise direction as viewed in FIG. 10. The cuttingangle A1 is negative when the inclination from the radial reference lineis in the clockwise direction as viewed in FIG. 10.

The magnitude of the cutting angle A1 has an influence on edge strengthof the prior art cutting tap. In this regard, one can increase thestrength of the cutting edge by reducing the cutting angle A1 (i.e.,making the cutting angle A1 more negative). However, while a reductionin the cutting angle A1 will increase the strength of the cutting edge,the amount of cutting force necessary to tap (or cut) the threadsincreases with the reduction in the cutting angle A1. When taps of theprior art are manufactured from cemented carbide, the cutting edges arevery prone to chipping because carbide has low strength as compared tohigh-speed steel. Specifically, the cutting edges that are most prone tochipping are the narrow edges on the chamfer that approach and includethe first full thread after the chamfer. The narrow full threads afterthe chamfer are also prone to chipping because they have a smallincluded angle. The wider edges on the entry part of the chamfer are farless prone to chipping because they are not as narrow as the cuttingedges of the full threads.

It should be appreciated that the above description of the obstaclesconnected with the cutting angle A1 of a cutting tap that has a straightcutting face also exist for a cutting tap that has an arcuate cuttingface. In this regards, for a cutting tap that has an arcuate cuttingface, a chordal hook angle corresponds to the rake angle A1 for thecutting tap with the straight cutting face. The chordal hook angle isdefined as the angle between a radial reference line between the majordiameter to the center of the cutting tap and a chord between the distalcutting edge and the minor diameter of the cutting tap.

As shown in FIG. 11, the cutting edges of the conventional cutting tapare prone to chipping, especially the narrow cutting edges on thechamfer that approach and include the first full thread after thechamfer (illustrated by the third chamfered thread in FIG. 11). Thewider cutting edges on the entry part of the chamfer are less prone tochipping (illustrated by the first and second chamfered thread). Priorart taps have a chamfer defined by a single straight line at a chamferangle A2 with respect to the axis of the tap. Because the chamfer isstraight, the thickness T1 of the sections of material removed by eachchamfered cutting edge remains constant.

Because taps are geometrically weak, especially the cutting edges, theyare prone to chipping. Because cemented carbide has lower strength thanhigh-speed steel, taps made from cemented carbide are more prone tochipping than taps made from high-speed steel. Therefore, it is notpossible to currently use taps made from cemented carbide in someapplications where high-speed steel taps can be used.

BRIEF SUMMARY OF THE INVENTION

Briefly, according to an aspect of the invention, there is provided acutting tap comprising a body having an axial forward end and an axialrearward end and a central longitudinal axis, the body having a flutedsection at the axial forward end, the fluted section including achamfered fluted section extending from the axial forward end of thebody and terminating at a first full cutting thread, the chamferedfluted section comprising a first cutting thread located a firstdistance from the axial forward end of the body and a second cuttingthread located a second distance from the axial forward end of the body,the second distance being greater than the first distance, wherein thechamfered fluted section is shaped such that a thickness of sections ofmaterial removed by the second cutting thread is smaller than athickness of sections of material removed by the first cutting thread.

According to another aspect of the invention, there is provided acutting tap comprising a body having an axial forward end and an axialrearward end and a central longitudinal axis, the body having a flutedsection at the axial forward end, the fluted section including achamfered fluted section extending from the axial forward end of thebody and terminating at a first full cutting thread, the chamferedfluted section comprising a first cutting thread located a firstdistance from the axial forward end of the body and a second cuttingthread located a second distance from the axial forward end of the body,the second distance being greater than the first distance, wherein aperipheral surface of the chamfered Doted section is non-linear suchthat the thickness of sections of material removed by the second cuttingthread is smaller than the thickness of sections of material removed bythe first cutting thread.

According to yet another aspect of the invention, there is provided acutting tap comprising a body having an axial forward end and an axialrearward end and a central longitudinal axis, the body having a flutedsection at the axial forward end, the fluted section including achamfered fluted section extending from the axial forward end of thebody and terminating at a first full cutting thread, the chamferedfluted section comprising a first cutting thread located a firstdistance from the axial forward end of the body and a second cuttingthread located a second distance from the axial forward end of the body,the second distance being greater than the first distance, wherein thefirst cutting thread forms a first chamfer angle with respect to thecentral longitudinal axis, and wherein the second cutting thread forms asecond chamfer angle with respect to the central longitudinal axis, thesecond chamfer angle being smaller than the first chamfer angle.

According to still yet another aspect of the invention, a method ofmaking a cutting tap comprises the steps of:

-   -   grinding a blank to form a threaded body portion at an axially        forward end of the cutting tap;    -   grinding one or more flutes in the threaded body portion to form        cutting edges;    -   grinding the threaded body portion to form a first cutting        thread and a second cutting thread, the first cutting thread at        a first distance from the axially forward end of the cutting        tap, and the second cutting thread at a second distance from the        axially forward end of the cutting tap; and    -   grinding a chamfer in the threaded body portion such that a        thickness of sections of material removed from the second        cutting thread is smaller than a thickness of sections of        material removed from the first cutting thread during a tapping        operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detailed,description is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is an isometric view of a exemplary embodiment of a spiral-flutedcutting tap of the invention;

FIG. 2 is a side view of an exemplary embodiment of a straight-flutedcutting tap of the invention;

FIG. 3 is a side view showing the profile of the axial forward portionfo the cutting tap of FIG. 2 including the chamfered fluted section andthe junction between the chamfered fluted section and the constantdiameter (or finishing) section of the cutting tap;

FIG. 4 is an enlarged view of the left side (as viewed in FIG. 3) of theside view of FIG. 3 illustrating an exemplary embodiment in which theouter periphery of the cutting teeth of the chamfered fluted section isformed with a radius R;

FIG. 5 is an enlarged view of the left side (as viewed in FIG. 3) of theside view of FIG. 3 illustrating an alternate exemplary embodiment inwhich the chamfered fluted section is formed with at least two sectionshaving different chamfer angles;

FIG. 6 is a cross-sectional view of the upper flute taken along line 6-6of FIGS. 4 and 5;

FIG. 7 is a cross-sectional view of the upper flute taken along line 7-7of FIGS. 4 and 5;

FIG. 8 is a cross-sectional view of the upper flute taken along line 8-8of FIGS. 4 and 5;

FIG. 9 is a cross-sectional view of one flute of a prior art cutting tapthat has a straight cutting face;

FIG. 10 is a cross-sectional view of the upper flute taken along line10-10 of FIG. 9; and

FIG. 11 is an enlarged view of the left side (as viewed in FIG. 9) ofthe side view of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a cutting tap 20 with spiral flutes is shownaccording to an embodiment of the invention. The cutting tap 20 has anelongate body 22 with an axial forward end 24 and an axial rearward end26. The cutting tap 20 has a cylindrical shank portion (bracket 28)adjacent to the axial rearward end 26 and a spiral-fluted portion(bracket 30) adjacent to the axial forward end 24.

The cutting tap 20 is operatively connected to a machine tool or thelike at the cylindrical shank portion 28 thereof The spiral-flutedportion 30 has a chamfered region beginning at the axial forward end 24and extending in an axial rearward direction therefrom. The chamferedregion joins a constant diameter (or finishing) region that extends inthe axial rearward direction terminating at the juncture with thecylindrical shank portion 28.

In regard to specific tapping applications, spiral flute taps with aright hand helix pull the chips out of the hole (right hand thread) andare effective in blind holes. Left hand spiral fluted taps direct thechip ahead of the tap (right hand thread) and are effective in throughholes.

Referring now to FIG. 2, there is shown a straight-fluted cutting tap 40according to an embodiment of the invention. The straight-fluted cuttingtap 40 has an elongate body 42 with an axial forward end 44 and an axialrearward end 46. The straight-fluted cutting tap 40 has a cylindricalshank portion (bracket 52) adjacent to the axial rearward end 46 and astraight-fluted portion (bracket 50) adjacent to the axis forward end44. In reference to a specific application, taps with straight flutesare effective in materials such as cast iron that produce a short chip.

Referring now to FIG. 3, there is shown the axial forward portion of thestraight-fluted portion 50 of the straight-fluted cutting tap 40. Thereis a chamfered fluted section (bracket 54) beginning at the axialforward end 44 and extending in an axial rearward direction therefrom.Chamfered fluted section 54 extends for a pre-selected distance shown bythe dimension “X” in FIG. 3. The chamfered fluted section 54 terminatesat the junction with a constant diameter (or finishing) fluted section(bracket 56). The constant diameter fluted section 56 begins at thejunction with the chamfered fluted section 54 and extends in an axialrearward direction until it terminates at the junction with thecylindrical shank portion 52.

The chamfered fluted section 54 has a series of V-shaped cutting threadswhere each cutting thread has a cutting edge. The distal cutting thread58 has a cutting edge 59 and is the most axial forward cutting thread.Distal cutting thread 58 is adjacent to cutting thread 62, which has acutting edge 63. Cutting thread 62 is adjacent to cutting thread 66,which has a cutting edge 67. Cutting thread 66 is adjacent to cuttingthread 68, which has a cutting edge 69. It will be appreciated that theconstant diameter (or finishing) fluted section 56 begins with thecutting thread 66 and extends in the axial rearward direction therefromuntil its junction with the cylindrical shank portion 52.

The chamfered cutting edge 59 of the distal cutting thread 58 is thestrongest of the cutting threads because it is wider than, and not asnarrow as, the cutting edges of the other cutting threads (for example,the cutting edges 63 and 67 of cutting threads 62 and 66, respectively).

Reducing the thickness of the sections (thickness times the width) ofmaterial removed by each cutting edge of chamfered fluted section 54 canreduce the forces imposed on the weaker cutting edges approaching thefirst full thread 70. One common way to accomplish this is to lengthenthe dimension “X” of chamfered fluted section 54. But there are manyapplications, especially when tapping blind holes, where the clearanceat the bottom of the hole is limited and therefore the dimension “X” ofchamfered fluted section 54 cannot be increased. It is desirable toreduce the dimension “X” of the chamfered fluted section 54 even on tapsfor through holes in order to keep the distance the tap must travel to aminimum.

According to the principles of the invention, the cutting tap 40 hasgreater resistance to chipping by reducing the forces imposed on therelatively narrower cutting edges of the chamfered fluted section 54that approach and include the first full cutting thread 66. In general,the principles of the invention are accomplished by shaping thechamfered fluted section 54 such that the thickness of the sections ofmaterial removed by the cutting edges approaching the first full cuttingthread 66 is smaller than the thickness of the sections of materialremoved by the relatively wider cutting edges of the most axial forwardcutting threads of the chamfered fluted section 54. Only the cutting tap40 will he discussed below for brevity, however it will be understoodthat the principles of the invention can also be applied to the cuttingtap 20.

The principles of the invention described above can be accomplished bymany different embodiments. Referring now to FIG. 4, one embodiment ofthe invention that accomplishes the principles of the invention is toform the peripheral surface of the chamfered fluted section 54 of thecutting tap 40 on a non-linear, curved line. For example, the peripheralsurface of the chamfered fluted section 54 may be having a radius, R.The shape of the curved line is such that the thickness T2 of thesections of material removed by the cutting edge 67 approaching thefirst full cutting thread 66 is smaller than the thickness T3 of thesections of material removed by the wider cutting edges 59, 63 of themost axial forward cutting threads 58, 62. Therefore, the force on thecutting edges approaching the first full cutting thread 66 will hereduced and the likelihood of chipping reduced. The radius R of thecurved line may vary along the curve in order to accomplish theprinciples of the invention.

FIG. 5 illustrates an alternative embodiment that also accomplishes theprinciples of the invention. In this embodiment, the chamfered flutedsection 54 is formed with two or more sections formed by straight lines(dashed lines) at different chamfer angles with respect to the centrallongitudinal axis Z-Z of the cutting tap 40 (i.e., the chamfer anglesare linear). Specifically, the chamfer angle of the last section of thechamfered fluted section 54 that approaches the first full cuttingthread 66 is smaller than the chamfer angle(s) of the one or more axialforward sections of the chamfered fluted section 54. As shown in FIG. 5,for example, the chamfered fluted section 54 is composed of two sectionswith lengths L2 and L3 in which the last section with length L2 isformed with a chamfer angle A3 that is smaller than chamfer angle A4 ofthe more axial forward section with length L3. With this construction,the sections of the material removed by the cutting edges approachingthe first full cutting thread 66 are smaller than the thickness ofsections of material removed by the wider cutting edges on the entrypart of the chamfered fluted section 54. Therefore, the force on thecutting edges approaching the first full cutting thread 66 is reduced,thereby reducing the likelihood of chipping. It will be appreciated thatthe invention is not limited by the number of sections of the chamferedfluted section 54 formed with different chamfer angles, and that theinvention can be practiced with two or more sections of differentlengths and chamfer angles, so long as the last section of the chamferedfluted section 54 that approaches the first full cutting thread 66 has achamfer angle that is smaller than the chamfer angles of the more axialforward sections.

FIG. 6 illustrates the cutting face for the cutting thread 58 in theaxial forward portion of the chamfered portion 54. Here, the cuttingface 72 is straight and has an orientation to present a positive cuttingangle A1. Cutting angle A1 is the included angle between the radialreference line G-G (Le., the line passing through distal cutting edge 59and the center 74 of the cutting tap) and a line H-H that lies along thecutting face 72. The cutting angle A1 is positive because the directionof inclination of line H-H relative to line G-G is in thecounterclockwise direction as view in FIG. 6. Because the cutting edgesare stronger in the axial forward section of the chamfered portion 54,they can utilize a positive cutting angle, which allows air an easiercutting action.

FIG. 7 illustrates the cutting face at the cutting thread 62, which islocated in a more axial rearward location than the cutting thread 58. InFIG. 7, the cutting face 76 presents a convex shape as defined bytransition radius R1. The length of transition radius R1 can varybetween about five percent to about one hundred percent of the diameterof the cutting tap. The cutting angle is the included angle between theradius reference line and a line (I-I) tangent to the cutting face atthe distal cutting edge 63, i.e., the axial forward termination of theconvex cutting face 76. Here, the cutting angle is zero degrees, andhence, only line I-I is referenced because line I-I is coextensive withthe radial reference line. The convex cutting face 76 also has an axialrearward termination 78. Line J-J is a line that is tangent to theconvex cutting face 76 at the axial rearward termination 76. Angle A1 isthe included angle between line I-I and line J-J and is equal to thecutting angle A1 shown in FIG. 6.

In constant diameter or finishing section of the chamfer and for threadspast the chamber such as, for example, the threads 70 shown in FIG. 8,the edges of the chamfer or full threads are weaker and prone tochipping. The cutting angle A2 is reduced because the threads 70 areweaker than the more axial rearward threads. Referring to thread 70,there is a convex-shaped cutting face 80 that defines a cutting angleA2, which is the included angle between the radial reference line (M-M)and a line (K-K) tangent to the cutting face at the distal cutting edge71. The cutting angle A2 is negative because the inclination of the lineK-K relative to line M-M is in the clockwise direction as viewed in FIG.8. It will he appreciated that the negative cutting angle compensatesfor the weaker thread 70 to optimize the overall tapping operation ofthe cutting tap. The convex cutting face 80 also has an axial rearwardtermination 82. Line L-L is a line that is tangent to the convex cuttingface 80 at the axial rearward termination 82. Angle A1 is the includedangle between line L-L and the line M-M and is equal to the cuttingangle A1 shown in FIG. 6.

The movement of the center point of the transition radius R1 relative tothe distal cutting edge allows a smooth transition from the positivecutting angle A1 in the axial forward section of the chamfered flutedsection 54 to the negative cutting angle A2. The geometry of the cuttingface as defined by the radial inward progressive movement of the centerpoint of the constant radius (R1) relative to the distal cutting edgeresults in cutting angles that are in between the positive cutting angleA1 and the negative cutting angle A2. Therefore, the cutting facegeometry of the inventive cutting tap is optimized to allow effectivecutting angles where needed on the forward entry part of the chamfer,and chip resistant cutting edges on later finishing portions of thechamfer and threads axial rearward of the chamfer. In regard to thecutting action of the cutting tap 40, the cutting tap 40 generates aninternal screw thread form by a succession of cutting edges on thechamfered section of the tap. Material is removed from the wall of thehole until the final thread form is obtained with the first full threadon the constant diameter fluted section 56. This progressive formationof an internal thread is shown in FIG. 6 by superimposing the sectionsof material removed by each of the four flutes.

In regard to ranges of the cutting angles, the cutting tap 40 made fromcemented carbide can be effectively used when angle A1 is within therange of about 5 degrees negative to about 15 degrees positive and theangle A2 is within the range of about 0 degrees to about 25 degreesnegative. The size of the radius R1 controls the transition from thecutting angle A1 to the cutting angle A2 by forming a chord between A1and A2 that ranges in width from about 0 percent to about 80 percent ofthe thread height. An exemplary chord N of a length P is shown in FIG.6.

It should be appreciated that the balance of the cutting tap fluteleading to the cutting face of the cutting tap 40 can take any shapeused in current practice as long as the radius of the flute is tangentto the line defined by angle A1.

Another option is to form the tap such that this profile remainsconstant along both the chamfer and the body of the tap past thechamfer. In this case, the cutting face angle at the cutting edges willbe A2 along the entire length. As the chip is formed starting at thecutting edge and flows across the cutting face, it will be first opposedby a low cutting angle A2 that transitions through the radius R1 to ahigher cutting angle A1.

In regard to the manufacture of the cutting tap, the cutting tap ismanufactured from a cylindrical blank composed of high-speed steel orsintered tungsten carbide, frequently referred to as a substrate. Theblank has a diameter that is sized larger than the finished dimensionsof the cutting tap and is cut to length.

The first step in processing the substrate is to grind the blank toprecision cylindrical tolerances by methods, such as cylindricaltraverse grinding on centers or be centerless infeed grinding methods.During this step, a cylindrical shank is ground to size at the axiallyrearward end of the tap and the major diameter of a threaded bodyportion is formed at the axially forward end of the tap. Additionallyduring this process, or as a consequence of an additional process, anoptional neck portion may be created with a cylindrical surface, and abevel between the cylindrical shank and the neck portion. Additionally,an optional bevel may be ground on the ends of the taps by cylindricalgrinding. In general, the shank diameter is approximately equal to thenominal thread diameter, but the shank diameter may be smaller than thenominal thread diameter for large diameter taps, and alternativelylarger for small diameter taps. An option may be the grinding of asquare as part of the shank at the extreme axially rearward end of thetap, as shown in FIG. 2.

In the next step, one or more flutes are ground so as to provide cuttingedges, in combination with the chamfer. The flutes may be straight orhelical, either right or left hand in any combination with either rightor left hand threads. As shown in FIG. 10, the cutting angle A1 may bebetween about 20 degrees negative for use in very hard materials toabout 20 degrees positive for very ductile materials.

Alternatively, the flute may be formed with a varying cutting face anglealong the length of the chamfer, as shown in FIGS. 6-8. The shape of thegrinding wheel is formed so as to provide a cutting face with theselected cutting angles A1 and A2, with A1 and A2 tangent to radius R1,where A1 is more positive than A2. The balance of the flute may beshaped according to current art, as long as A1 is tangent to a radiusleading to the balance of the flute. The complete form may be ground inone or two steps. For example, the flute may be ground in two steps byfirst grinding the flute according to current art, and then grinding theinvented cutting face in a following operation. Alternatively, the wheelmay he shaped so as to generate the complete form in one operation.

In the next step, the threaded body portion is ground to form theV-shaped thread flank surfaces, along with minor and major diameters, onthe helix. Subsequently, the shape of a threaded cutting chamfer portionis formed by grinding. The V-shaped thread flank surfaces and majordiameter replicate the internal screw thread that is generated duringtapping.

The cutting chamfer portion is ground with a taper so as to allow entryin the hole to be tapped. The chamfer may be ground either to form thechamfer on a curved line as shown in FIG. 4, or by forming a chamferwith two or more sections formed by straight lines at angles to the axisof the tap such that the chamfer angle of the last section thatapproaches the first full cutting thread is smaller than the chamferangle of the first section, as shown in FIG. 5. By either method, thesections of material removed by the cutting edges approaching the firstfull cutting thread are smaller than the thickness of sections ofmaterial removed on the entry part of the chamfer.

The length of the chamfer may be as small as one (1) thread pitch fortapping blind holes to as long as fifteen (15) thread pitches whentapping very hard materials. The number of chamfer sections each with adifferent angle (FIG. 5) will depend on the overall length of thechamfer and will increase in number as the overall chamfer lengthincreases.

After the chamfer is ground, the effective cutting edge angle is A1 withthe first entry portion of the chamfer and gradually progresses tocutting angle A2 in later finishing portions of the chamfer. Thiscombination will reduce the likelihood of chipping by not only reducingthe force on the cutting edges approaching the first full cuttingthread, but also by increasing the strength of the same edges byreducing the cutting face angle.

After grinding, the tap may be honed with abrasive media or abrasivebrushes so as to form a small radius on the cutting edges and othersharp corners. The resulting radius may be between about 0 microns andabout 100 microns. This honing further increases the strength of theseedges.

As a final step in the process, the tap may be optionally coated with awear resistant layer (not shown) of metal nitrides, carbides,carbonitride, borides and/or oxides, wherein the metal is chosen fromone or more of the following: aluminum, silicon and the transitionmetals from Groups IVa, Va and VIa of the Periodic Chart. This layer isdeposited as a single monolayer or in multiple layers, includingalternating layers. Low friction layers can also be deposited on top ofthese wear resistant layers.

As can be appreciated, the invention provides a cutting tap that allowsfor the use of a cemented carbide cutting tap that is not prone tochipping. The use of a cemented carbide cutting tap possesses a numberof advantages as compared to a tap made of “high-speed” steel. Forexample, the cemented carbide cutting tap results in an improvement ofthe dimensional accuracy with respect to the size and shape of thethreads as compared to high speed steel cutting taps. In addition, acemented carbide cutting tap results in an increase in the useful toollife of the cutting tap as compared to high speed steel cutting taps.Further, a cemented carbide cutting tap increases the production speedfor internal screw threads as compared to a high speed steel cuttingtap.

The documents, patents and patent applications referred to herein arehereby incorporated by reference.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation, and the scope of theappended claims should be construed as broadly as the prior art willpermit.

1-17. (canceled)
 18. A method of making a cutting tap, comprising thesteps of: grinding a blank to form a threaded body portion at an axiallyforward end of the cutting tap; grinding one or more flutes in thethreaded body portion to form cutting edges; grinding the threaded bodyportion to form a first cutting thread and a second cutting thread, thefirst cutting thread at a first distance from the axially forward end ofthe cutting tap, and the second cutting thread at a second distance fromthe axially forward end of the cutting tap; and grinding a chamfer inthe threaded body portion such that a thickness of a section of materialremoved from the second cutting thread is smaller than a thickness of asection of material removed from the first cutting thread during atapping operation.
 19. The method according to claim 18, wherein thechamfer is non-linear such that a peripheral surface of the first andsecond cutting threads is on a curved line.
 20. The method according toclaim 18, wherein the chamfer is formed such that a chamfer angle of thesecond cutting thread is smaller than a chamfer angle of the firstcutting thread.